CAM Cycle (Crassulacean Acid Metabolism)

The specialized photosynthetic pathway known as “crassulacean acid metabolism” (CAM) enables plants to flourish in arid environments by minimizing water loss through the temporal separation of CO2 uptake and fixation. CAM plants, like cacti and pineapple, fix carbon dioxide at night when temperatures are lower and humidity is higher, storing it as malice acid. During the day, the stored malice acid is decarboxylated, and the released CO2 is used for photosynthesis with stomata closed, minimizing water loss.

Here’s a more thorough explanation of how CAM works:

• Nocturnal CO2 Uptake: At night, CAM plants open their stomata and absorb CO2 from the atmosphere.
• Carbon Fixation: The enzyme phosphoenolpyruvate carboxylase (PEPC) transforms the absorbed CO2 into organic acids, primarily malice acid.
• Malice Acid Storage: The malice acid is then stored in the plant’s vacuoles until daytime.
• Daytime Photosynthesis: When sunlight becomes available, the stomata close, and the stored malice acid is broken down, releasing CO2.
• CO2 Utilization: The released CO2 is used by the Calvin cycle for photosynthesis, producing sugars and other carbohydrates.

Key Advantages of CAM:

• Water Conservation: By taking up CO2 at night and closing stomata during the day, CAM plants minimize water loss through transpiration.
• Reduced Photorespiration: CAM plants can concentrate CO2 around the enzyme Rubisco, reducing photorespiration (a wasteful process that occurs in C3 plants).
• Adaptation to Arid Environments: CAM is particularly well-suited for plants in hot, dry environments where water is scarce.

Examples of CAM Plants:

Cacti, pineapple, agave, orchids, and bromeliads.

In essence, CAM is a clever adaptation that allows plants to thrive in challenging environments by decoupling CO2 uptake from photosynthesis, leading to significant water conservation.

CAM Photosynthesis

CAM pathway is adapted in plants to perform photosynthesis under stress. The CAM pathway reduces photorespiration.

In CAM plants stomata are open at night and they absorb carbon dioxide at night to reduce water loss during the daytime. The process has the following steps:

1. The first step in carbon dioxide fixation is the combination of CO₂with PEP (phosphoenolpyruvate) to form 4 carbon oxaloacetate (same as C₄ plants) in the chloroplast of mesophyll cells.
   The reaction is catalysed by PEP carboxylase. This occurs at night.
2. Oxaloacetate is converted to malate and other C₄ Malate is stored in vacuoles at night.
3. During the daytime, stomata remain closed, so there is no gas exchange. Malate is transported out of the vacuole and CO₂ is released by the process of decarboxylation.

This CO₂ finally enters the Calvin cycle and carbon fixation completes. The CO₂ which gets accumulated around RuBisCO increases the efficiency of the photosynthesis process and minimizes photorespiration.

CAM Plants Examples

The majority of CAM plants are xerophytic. Some water plants, like Vallisneria and Hydrilla, also include the CAM pathway. The CAM pathway in aquatic plants is triggered by CO2 shortage. Slower diffusion in water limits the supply of carbon dioxide. At night, when there is less competition from other photosynthetic plants, aquatic CAM plants take up CO2.
A few typical instances of CAM plants
A few species of Euphorbia and Bromelioideae, as well as orchids, cacti, aloe, pineapples, agave, and moringa, are among the plants.

Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions (e.g., cacti) or in tropical epiphytes (e.g., orchids and bromeliads). CAM plants close their stomata during the day and take up CO₂ at night, when the air temperature is lower and water loss can be lowered by an order of magnitude. CAM occurs in between 5% and 10% of plants and is always associated with succulence, at least at a cellular level.

Although the biochemistry of CAM plants is similar to that of C4 plants, the two carboxylations are now separated in time rather than in space. Malic acid is synthesized from carbohydrates, via PEP carboxylase, at night, and is stored in the vacuole. During the day, malate is decarboxylated, via PEP carboxykinase or NAD(P)-malic enzymes
in the cytosol, and CO₂ is re-fixed via the Calvin- Benson cycle and carbohydrates are reformed. This process occurs behind closed stomata. The internal concentration of CO₂ is raised as high as 10,000 ppm, which also suppresses photorespiration. CAM plants show a high degree of metabolic flexibility. Seedlings and well- watered plants may show little or no CAM and perform C3 photosynthesis, opening their stomata during the day. This allows increased carbon gain during periods of water availability or during seedling establishment. Water or salt stress can then induce CAM, switching on gene expression for synthesis of the component enzymes.

Min Chen is a professor of plant biochemistry and molecular biology in the School of Life and Environmental Sciences at the University of Sydney, Australia. Her principal research interests are molecular mechanism of photosynthesis driven by long-wavelength light. She holds a PhD degree from the University of Sydney. She led the team to discover the 5th chlorophyll, chlorophyll f, which has the most red – shifted absorption from the oxygenic photosynthetic organisms.

Robert E. Blankenship is the Lucille P. Markey Distinguished Professor of Arts and Sciences, Emeritus, from WashingtonUniversity in St. Louis, USA. His principal research interests are: excitation and electron transfer in photosynthetic systems and the origin and early evolution of photosynthesis. He obtained his PhD degree from the University of California, Berkeley.